The invention relates to a method for the production of a heat storage means, more particularly in the form of a latent heat storage means, for vehicle heating systems run on heat from the engine, comprising a housing, which is made up of an outer container and an inner container arranged in spaced relationship to the outer container so as to constitute an insulating vessel including an insulating zone, a heat storage core arranged in the inner container and having at least one chamber for a heat storage medium in it, said chamber being separated by a partition wall from at least one flow path for a heat transfer medium, and an inlet duct and outlet duct for the heat transfer medium, such ducts being connected with the flow path and extending outwards through the insulating zone, the insulating zone having been baked out in the housing for degassing and having been evacuated, and between the filling temperature of the storage when being being charged into the storage zone, and the maximum temperature occurring thereafter the volume of the storage medium changes with temperature to a greater extent than the respective chamber volume.
During the production and the operation of a heat storage means temperature thresholds have to be adhered to, which are dependent on the selection of the material employed. In this respect it may sometimes be a question of the materials used for the components; more particularly however such temperature thresholds are set with respect to the heat storage medium. For instance, the heat storage medium may be comparatively sensitive to thermal effects and be inclined to degrade when a threshold is exceeded. However furthermore the action on the heat storage means, more particularly under the influence of vapor pressure and thermal expansion, is significant.
These problems are more particularly explain taking a latent heat storage means as an example. In the case of heat storage means a heat storage medium is employed, which in the charged state of the heat storage means is liquid and below a so-called transition temperature in the discharged condition of the heat storage means is solid. The operational temperature of the heat storage means is predetermined by the temperature of the heat transfer medium utilized for charging the heat storage means and hence for the field of use of the heat storage means. If in the case of motor vehicles the cooling water is utilized as the heat transfer medium, the operational temperature of the heat storage means will generally be at 90.degree. C., a maximum temperature of 125.degree. C. having to be taken into account. If Ba(OH).sub.2.8H.sub.2 O is employed as the heat storage medium, whose boiling amounts to 110.degree. C., then at the maximum operational temperature a vapor pressure of regenerator 1.5 bar is to be expected. When filling the heat storage zone with the heat storage medium the temperature thereof should be somewhat under the fusion point in order to ensure the simplest possible handling in the liquid condition. There is thus a temperature difference between the filling temperature and the maximum operating temperature, this being likely to be the cause of substantial thermal expansion of the storage medium, which in addition to the vapor pressure leads to a mechanical loading of the storage core.
A further temperature which has to be taken into account is the bake out temperature occurring during the manufacture of the heat storage means. After the mechanical manufacture of such a storage housing the desired insulating effect is able to be achieved in a matter of minutes by evacuation of the insulating zone. However, this insulating action is not permanent, since materials, which may evaporate in the course of time and thus increase the pressure in the insulating zone and therefore degrade the insulating effect of the vacuum, are absorbed on the surfaces of the outer and of the inner container adjacent to the insulating zone, on the surfaces of any radiation shields located in the insulating zone, or on microporous insulating materials, and also on the obligatory internal structures in the insulating zone, that is to say the supply and removal ducts and the means for mounting the inner container.
For this reason after mechanical processing during manufacture vacuum insulated vessels are degassed for a prolonged time, the insulating zone being continuously exhausted by a pump. In order to reduce the time required for degassing to an economic level, as for example to 24 hours, the insulating vessel is heated to an elevated temperature during degassing, this being termed bake out. It is to be seen form experience that an increase in temperature of 10.degree. C. halves the degassing time.
It is furthermore known that a major contaminant of vacuum vessels is water deposited on the walls. For the removal of such water there are three pronounced temperature stages, that is to say approximately 120.degree. C. with low evaporation rates, 180.degree. C. with very high evaporation rates and approximately 360.degree. C. with an evaporation which is practically increased one hundredfold.
It is also known that during degassing the long-term effect depends on the lowest temperature, which is reached at the surfaces in the insulating zone. It follows from this that for a certain long-term effect all surfaces defining the insulating space have to reach or exceed the minimum temperature.
For large-scale use in automobile engineering a long-term effect of the vacuum is necessary, which calls for bake out at the second temperature level of about 180.degree. C., a value which is above the boiling point of most heat storage media so that the bake out temperature has to be substantially above the maximum operational temperature, for which reason--if the storage medium is exposed to this temperature--the storage core would be exposed to the effects of a vapor pressure which is still further increased and to a thermal expansion which is also increased.
The vapor pressure occurring at the maximum temperatures reached may be coped with by suitable dimensioning and design of the heat storage means components exposed to this vapor pressure.